It is well known that there are numerous problems associated with processes for precipitating dust from hot dust-laden exhaust gases from rotary kiln plants.
Usually electrostatic precipitators are used in such processes. In order to obtain thorough cleaning of the exhaust gases, various agents are used to condition the gases and to reduce the resistivity of the dust so that an improved performance of the electrostatic precipitator can be obtained. Cleaning efficiencies can be above 99 percent. The exhaust gases can be conditioned by injecting water or steam into them with or without various chemicals so that the resistivity of the dust is reduced to an acceptable level.
It is known to condition exhaust gases from the burning of mineral materials, especially cement raw materials to cement clinker, in rotary kilns. This is accomplished by dosing or injecting into the gases one or more water-soluble alkali-metal compounds as a conditioning agent before the gases are passed to an electrostatic precipitator.
This conditioning has a remarkable effect upon the resistivity of the dust. Resistivity is reduced to a level often below 10.sup.10 ohm cm. thereby making it possible to increase the filter current in and thus the efficiency of the precipitator.
When using solid fuel in rotary kilns, large amounts of fly ash are created and carried with the exhaust gases to the electrostatic precipitator. The particle size distribution of the fly ash in the exhaust gases is often such that the mean diameters of the particles are between 10 and 20 microns. These particles are difficult to precipitate and their resistivity often exceeds a critical value of about 10.sup.10 ohm cm. Use of low sulphur content coals, preferred in recent years to avoid air pollution by sulphur oxides, result in a resistivity above the critical value.
According to recent investigations, the presence of sodium in the ash, in amounts of 2 percent as Na.sub.2 O, reduces the resistivity of the fly ash from low sulphur coals below the critical value.
It is known to inject water-soluble alkali-metal compounds as a conditioning agent into the exhaust gases before they are passed to the precipitator. This serves to reduce the resistivity of any fly ash in the exhaust gases. However, by injecting the conditioning agents into the exhaust gases, it is not always possible to obtain regular distribution of the agents because the fly ash particles are extremely small and tend to remain in suspension as they pass through the precipitator. To obtain good conditioning results, multiple injection nozzles and injection chambers are required. This equipment complicates precipitator installation.
I have invented a method of conditioning exhaust gases from burning processes in which the resistivity of the exhaust gases is reduced without requiring complicated precipitator installations while improving performance of the electrostatic precipitator used in cleaning the gases. Thus, I have invented a method that overcomes the disadvantage of the prior art.